Actually, the Thunderbolt has an MDM 9600 baseband chipset that supports Hspa and Hspa+ which is a technology that At&t and T-mobile uses for their data connections. So, I believe that if the LTE radio is backwards compatible with those frequencies of At&t or T-mobile that it might work. The thunderbolt has two antennas: one for strictly CDMA and the other for lte and backwards compatibility to gsm/hspa(+) from what I understand. From what I have read Verizon has just opted to not install the drivers to support the GSM/HSPA(+) side of things. That, and the MSM 8650 processor or other hardware in the Thunderbolt may not support it. Just because the modem supports GSM/HSPA(+) doesn't mean the phone will, but I do think that if someone really wanted to they could modify the thunderbolt to work on GSM. The may need more hardware, but I do not know.
I would post the link about the chipset I have, but sadly as a noob it will not let me.
P.S. Verizon allows you to log-in to your account and view the unlock code for the thunderbolts SIM yourself. You do not have to call them. I have not used another Carriers SIM in my Thunderbolt, but HAVE read about people who have in other threads. Verizon from what I understand and have read on other forums does not allow you to use USA SIM cards other than VERIZON'S own SIM. From what I have read, it only unlocks the phone to use foreign country SIMs only. From what I had read, once you use VERIZON'S SIM unlock procedures a NEW menu pops up in your network settings giving you the option to select to use GSM only. The same thread was speaking about the iphone 4s and the Thunderbolt and said that Verizon allows for both to be unlocked and used as World Phones. I will not know for sure until I see proof, but I do know that Verizon allows you to unlock the Thunderbolt's SIM to use on Foreign Networks.